Well plug and abandonment choke insert

ABSTRACT

A process for plugging a wellbore comprises installing a choke device ( 1 ) into the wellbore to increase the back pressure allowing better control when introducing cement to plug the wellbore. The device ( 1 ) has a cylindrical shape with a central through bore ( 6 ). On the external profile is a shoulder ( 4 ) which, when the device is installed in a wellbore, comes to rest against a nipple in the wellbore. The device ( 1 ) may free fall for all or part of the way down but would normally be pumped down at least for the last part of its delivery. In the bore ( 6 ) is a pump out ball ( 10 ) which blocks the through bore until the device reaches the nipple and stops, at which point pressure increases and the ball is ejected. Cement and other liquid may then be pumped through the choke device to the distal region of the well.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a non-provisional application which claims thebenefit of and priority to U.S. provisional application Ser. No.61/447,789 filed Mar. 1, 2011, entitled “Well plug and abandonment chokeinsert,” which is hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

This invention relates to a method and apparatus for plugging a wellbore(such as e.g. an oil or gas well), or for preparing wellbore to beplugged, e.g. when it has reached the end of its productive life. Theinvention also relates to a plugged wellbore.

BACKGROUND OF THE INVENTION

When an oil or gas well is no longer economical or if there is someproblem with the well which means that production is no longer possibleor that well integrity has been compromised in some way, or for otherreasons, the well may be abandoned. It is common practice to plug thewell before abandoning it, e.g. to prevent seepage of hydrocarbonproduct from the well. This can also apply to water injectors, i.e.bores which have been drilled in order to pump water into a reservoir toincrease bottom hole pressure.

Commonly, plugging may be achieved by injecting a settable substance ormedium, e.g. cement, into the well. A well will normally have productionperforations, that is to say apertures in a well liner or casing throughwhich hydrocarbon product enters from the rock formation and travels tothe surface. During plug and abandonment operations it is common to seal(“squeeze”) production perforations with cement or another settablemedium which may then form a permanent barrier to flow across theperforations and out of the well.

The plugging process often involves pumping a surfactant liquid, knownas a “spacer”, into the well. The purpose of the spacer is to remove oilresidues from the internal surface of the well casing and/or liner androck matrix making them “water wet” (allowing better adhesion bycement). Commonly, immediately following the spacer, cement is pumpeddown the well to occupy the part of the well casing and/or liner whereperforations are to be squeezed. When sufficient cement has been pumpeddown, more spacer liquid and possibly other liquids may be pumped downthe well in order to place the cement at its final designed location.

It is desirable to be able to monitor with a reasonable degree ofaccuracy where the different constituents of the liquid column arelocated at any given time and the associated surface pumping (treating)pressure. It is also desirable to be able to control the progress of theliquid column, and other things, by varying pressure on the columnapplied at the surface. For these things to be achieved, it is helpfulto have a continuous column of liquid being pumped into the well.

It is therefore desirable to have sufficient reservoir pressure enteringthe well (“bottom hole pressure”) to support a standing column ofrelatively high specific gravity material, e.g. spacer liquid, cementand displacement fluid, reaching to the top of the well. The spacer andcement and other liquids may then be pumped down against this pressureand thereby an accurate determination of each constituent's location bemade at any given point within the process.

In many cases, the bottom hole pressure is insufficient to support astanding column of relatively high specific gravity liquid reaching tothe top of the well. In this event, positive pressure against the fluidcolumn at the surface cannot be maintained as liquids are introducedinto the top of the well. This results in liquid free falling down thewellbore and out through the reservoir completion, i.e. the perforatedsection of casing/liner.

In this situation, it is often not possible to monitor when the cementhas reached the desired wellbore location with respect to theperforations which it is desired to seal. Without an accurateunderstanding of where the cement is, it is possible to over-displacethe cement by continuing to introduce fluid at the surface whichfreefalls and over displaces the cement, with the result that theproximal perforations are not effectively squeezed. Alternatively, it ispossible to under-displace the cement thereby leaving distalperforations unplugged and at the same time creating a barrier in themore proximal part of the liner hindering further optimized pluggingoperations within the wellbore without revision to procedures.

In the past, attempts have been made to address this issue by theaddition of solid plugging material to the liquid plugging fluid or bydisplacement of the liquid plugging fluid with a low specific gravityfluid.

The addition of solid plugging material partially closes off pathways atthe perforations creating backpressure or the need for additionalpumping pressure at the surface in order to displace the plugging to thedesired location. Thus a positive pressure on the fluid column ismaintained at the surface. Partly closing perforations with solidmaterial can be undesirable since the perforations can end up notadequately plugged. Conversely, because there is little control over thedegree of plugging and at which point it will occur, an undesirableoutcome can result if all perforations are plugged off with medium priorto achieving designed displacement.

Similarly, displacement of the plugging fluid with a significantly lowerspecific gravity fluid may also allow positive pressure to be maintainedat the surface. However, there is a limit in available low specificgravity fluids and the constituents incorporated in them that meetdesign requirements.

U.S. Pat. No. 6,520,256 discloses a choke for use in cementing casingsin oil and gas wells.

BRIEF SUMMARY OF THE DISCLOSURE

The invention more particularly includes a process for plugging awellbore wherein the process comprises the steps of (a) installing achoke device into the wellbore, the device having a through bore with apredetermined diameter. The invention is particularly applicable whenthe bottom hole pressure of the wellbore is insufficient to support astanding column of liquid of specific gravity 1 reaching to the surface.

The choke device may be installed by allowing it to free fall down thewell or it may be pumped down the well, or a combination of the two. Ifthe wellbore includes a nipple, the choke device may bottom out at thenipple.

In one embodiment, the choke device includes a pump out valve. When thedevice reaches the nipple and stops, pumping is continued therebyincreasing pressure until the pump out valve is actuated, therebyallowing flow of fluid through the device. The pump out valve may bethought of as a device for blocking the through bore until apredetermined pressure is applied e.g. on the proximal side of thevalve. The valve may be an arrangement as simple as a deformable plug ofspherical or other shape which is located in the through bore betweentwo shoulders; when the pressure increases beyond a certain level theplug is forced out thereby opening the bore for liquid to flow throughit.

If a christmas tree is located at the top of the wellbore, the step ofinstalling the choke device may include inserting the device between theswab valve and the master valve (top or bottom master valve) and thenopening the necessary valves below the device to gain access to thewellbore.

To plug the wellbore, settable medium (such as cement) may be injectedthrough the choke device into the distal part of the wellbore, that isto say the part beyond the choke device. Surfactant may be injected intothe wellbore before the settable liquid, and another liquid may beinjected after. The pressure at the surface would normally be monitoredin which case it may be possible to determine from the monitoredpressure when the settable medium reaches the choke device.

The choke device itself may comprise a body capable of being passed downa wellbore. The body may have an external profile including a shoulder,a through bore and a pump out valve which blocks the through bore untila predetermined pressure is applied to the valve. The pump out valve maybe a ball retained in the through bore by a stop surface. Seals may beprovided on the external profile of the body.

A method of preparing a wellbore for plugging may comprise insertinginto the wellbore a choke device as described above. The method mayinclude applying pressure to the choke device thereby pumping the devicethrough the wellbore and subsequently increasing the pressure to a levelat which the pump out valve is actuated and the through bore unblocked.This preferably happens when the device has reached a nipple in thewellbore, e.g. adjacent a production packer.

The invention also relates to a plugged wellbore having located in it achoke device as described above.

The term “wellbore” as used herein shall be taken to mean an oil or gaswell or a water injector.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention and benefitsthereof may be acquired by referring to the following description takenin conjunction with the accompanying drawings in which:

FIG. 1 is a side view of a choke insert in accordance with theinvention;

FIG. 2 is a side view, partly in section, of a well christmas treeshowing the choke insert of FIG. 1 in place; and

FIG. 3 is a side view, partly in section, of part of a wellbore showingthe choke insert of FIG. 1 located against a nipple in the wellbore.

DETAILED DESCRIPTION

Turning now to the detailed description of the preferred arrangement orarrangements of the present invention, it should be understood that theinventive features and concepts may be manifested in other arrangementsand that the scope of the invention is not limited to the embodimentsdescribed or illustrated. The scope of the invention is intended only tobe limited by the scope of the claims that follow.

FIG. 1 shows a choke insert 1. It comprises a cast body of aluminumhaving a generally cylindrical shape. Its outer profile includes aproximal portion 2 and a distal portion 3 of reduced diameter comparedwith the proximal portion. The words “proximal” and “distal” relate tothe orientation of the device when in place in a well: the proximalportion being closer to the surface and the distal portion closer to theend of the well.

Between the proximal and distal portions is a shoulder 4. On the outerprofile in the reduced diameter distal portion are two seals 5. Thefunction of the seals 5 is to restrict fluid flow around the outside ofthe insert 1 such that all flow is directed through the device.Extending through the device is a bore 6 having a diameter whichdecreases towards the distal end of the device; at its most distal end,the bore 6 has a diameter 7 as shown in FIG. 1. The bore 6 is providedwith a flared proximal end 8, to help to reduce turbulence within thefluid flow regime.

An end plug 11 is screwed into the distal end of the bore of the insert1 which provides an internal shoulder or stop surface 12. The end plughas a bore 13 of the same diameter 7 as the most distal part of the bore6 through the main body of the choke insert 1. The bore 13 also has aflared end 9 to help reduce turbulence.

Located on the proximal side of the end plug 11 is a ball 10 of Phenolicplastic which sits adjacent the stop surface 12. The ball 10 is retainedbetween the stop surface 12 and the distal end of the bore 6 in the mainbody. The dimensions and elastic modulus of the ball are carefullyestablished, relative to the diameter 7, such that the ball will deformand pass the stop surface 12 if a predetermined pressure is applied tothe proximal side of the ball 10.

FIG. 2 shows the christmas tree valve arrangement at the top of awellbore to be plugged. The length of the choke insert 1 is chosen sothat it will fit into the christmas tree 20 between the swab valve 21and the upper master valve 22. The overall outer diameter is chosen sothat it will pass down the well to a point just above the reservoir. Thedimensions of the distal portion 3 and the shoulder 4 are chosen so thatthe choke insert device 1 will rest against a nipple 31 (see FIG. 3) inthe wellbore 30 in region of the top of the reservoir, near theproduction packer (not shown).

Before plugging a well, sea water is pumped down the well to determinewhether it is possible to inject liquid into the perforations in theproducing part of the well (not shown) which it is desired to squeeze(block with cement or other settable plugging fluid). A gage device, ofdiameter and length determined by the choke insert and the tubular beingtraversed, is also lowered into the well to determine whether anyobstructions are present which may obstruct and prevent the correctpositioning of the choke insert.

This having been done, the bottom master valves and swab valve areclosed and any trapped pressure is bled off between them at the wingvalve 23. The swab valve is then opened and the choke insert 1 is theninserted into the christmas tree 20, the swab valve 21 closed, and thenthe bottom and upper master valves are 24, 22 opened. The choke insert 1then free falls down the well. In a simple vertical well, the choke mayreach the desired location (nipple 31) without pumping. Normally,however, the friction will be too great and/or the well has an inclinedor horizontal portion and the insert has to be pumped. Sea water isnormally used for this purpose. Pumping continues until an increase inback pressure is noted which is indicative of the choke having come torest at the profile nipple 31. Pumping is continued until the pressurerises sufficiently to displace the ball 10 past the stop surface 12. Asudden drop in monitored pressure indicates that the ball has beendisplaced. Step rate tests are then conducted to determine the backpressure created by the choke at increasing injection rates. This datais used to refine the predicted surface treating pressure profile whichshould be experienced during the placing of the plugging fluid.

Spacer fluid, a specialized mixture of chemicals including surfactants,is then pumped down the well at a rate sufficient to maintain positivepressure and contact with the top of the fluid column. The choke insert1 with its small diameter bore, allows the fluid column to maintain thepositive back pressure. Cement is then delivered in a continuous liquidcolumn directly following the spacer fluid. A further liquid or liquids,e.g. further spacer fluid or other liquids, follow the cement in acontinuous liquid column. Pressure continues to be applied to the cementvia this liquid column, and monitoring of back pressure continues.

As the spacer/cement interface passes the choke insert 1, a pressurechange may be recorded at surface due to its unique viscosity anddensity. The volume of the wellbore tubulars can be determined as wellas the volume of the perforated liner, so the volume of cement pumpeddown the well can be controlled to give the best chance of filling theperforated liner with cement. When the other end of the cement columnpasses the choke insert 1, a further change in surface pressure may benoted due to its unique viscosity and density. The aim is to squeeze allthe perforations but not to over- or under-displace the cement column.

In closing, it should be noted that the discussion of any reference isnot an admission that it is prior art to the present invention,especially any reference that may have a publication date after thepriority date of this application. At the same time, each and everyclaim below is hereby incorporated into this detailed description orspecification as an additional embodiments of the present invention.

Although the systems and processes described herein have been describedin detail, it should be understood that various changes, substitutions,and alterations can be made without departing from the spirit and scopeof the invention as defined by the following claims. Those skilled inthe art may be able to study the preferred embodiments and identifyother ways to practice the invention that are not exactly as describedherein. It is the intent of the inventors that variations andequivalents of the invention are within the scope of the claims whilethe description, abstract and drawings are not to be used to limit thescope of the invention. The invention is specifically intended to be asbroad as the claims below and their equivalents.

1. A process for plugging a wellbore, wherein the process comprises thesteps of: a) installing a choke device into the wellbore, the devicehaving a through bore with a predetermined diameter; b) injecting asettable medium into the wellbore.
 2. The process according to claim 1wherein the bottom hole pressure of the wellbore is insufficient tosupport a standing column of liquid of specific gravity 1 reaching tothe surface.
 3. The process according to claim 1 wherein the step ofinstalling the choke device comprises allowing the choke device to freefall down the wellbore.
 4. The process according to claim 1 wherein thestep of installing the choke device comprises pumping the choke devicedown the wellbore.
 5. The process according to claim 1 wherein thewellbore includes a nipple and wherein the step of installing the chokedevice comprises having the choke device bottom out at the nipple. 6.The process according to claim 5 wherein the choke device includes apump out valve and the step of installing the choke device comprisespumping the device down the wellbore until it bottoms out at the nippleand then continuing to pump thereby increasing pressure on the deviceuntil the pump out valve is actuated.
 7. The process according to claim1 wherein located at the top of the wellbore is a christmas tree with aswab valve and a master valve and wherein the step of installing thechoke device comprises inserting the choke device between the swab valveand the master valve and then opening the master valve to allow thedevice to enter the wellbore.
 8. The process according to claim 1wherein a proximal part of the wellbore is defined between the chokedevice and the surface and a distal part of the wellbore is definedbetween the choke device and the distal end of the wellbore and whereinthe step of injecting a settable medium comprises injecting some or allof the settable medium through the choke device into the distal part ofthe wellbore.
 9. The process according to claim 1 further comprising thestep of injecting a surfactant liquid into the wellbore prior toinjecting the settable medium.
 10. The process according to claim 9further comprising the step of injecting a third liquid after injectingthe settable medium.
 11. The process of claim 10 comprising monitoringpressure at the surface of surfactant liquid, settable medium and thirdliquid.
 12. The process of claim 11 further comprising determining fromsaid monitored surface pressure when the settable medium reaches thechoke device.
 13. A choke device for use in a process for plugging awellbore, wherein the device comprises a body capable of being passeddown a wellbore, wherein the body has: a) an external profile whichincludes a shoulder; b) a through bore; c) a pump out valve which blocksthe through bore until a predetermined level of pressure is applied tothe valve.
 14. The choke device according to claim 13 wherein thethrough bore includes a stop surface and wherein the pump out valvecomprises a ball retained in the through bore by the stop surface. 15.The choke device according to claim 13 wherein one or more seals areprovided on the external profile of the body.
 16. A method of preparinga wellbore to be plugged comprising inserting into the well a chokedevice, wherein the device comprises a body capable of being passed downa wellbore, wherein the body has: a) an external profile which includesa shoulder; b) a through bore; c) a pump out valve which blocks thethrough bore until a predetermined level of pressure is applied to thevalve.
 17. The method according to claim 16 further comprising the stepsof: (i) applying pressure at a first level to the choke device, saidpressure being below said predetermined pressure level, thereby pumpingthe device through the wellbore; and (ii) subsequently applying pressureto the choke device at a second level at or above said predeterminedlevel, whereby said pump out valve is activated and the through boreunblocked.
 18. The method according to claim 17 wherein, in step a), thechoke device is pumped through the wellbore until it reaches a nipple.19. The method according to claim 18 wherein the nipple is adjacent aproduction packer of the wellbore.
 20. A plugged wellbore having locatedtherein a choke device, wherein the device comprises a body capable ofbeing passed down a wellbore, wherein the body has: a) an externalprofile which includes a shoulder; b) a through bore; c) a pump outvalve which blocks the through bore until a predetermined level ofpressure is applied to the valve.